
I find the 24 is plenty wide for how I shoot with my Xti. Being able to shoot up to 105 is so much nicer than just 55.
for the XTi, you might also consider a Tamron SP 24-135. i have one that i've used on my 30D and 40D with great results; my most successful image (see it here) was taken with that lens and my 30D. it may not be quite up to the 24-105 in terms of optical quality, and doesn't have IS, but it's a third the price, and that counts for a lot when you're saving up for a new camera body. it provides excellent image quality on a crop body, and also gives you a bit more on the long end than the 24-105...